[PATCH 1/2] sha1_name.c: signal if @{-N} was a true branch nameor a detached head

[Junio C Hamano <gitster@xxxxxxxxx>]

The original API read "checkout: moving from (.*) to ..." from the
reflog of the HEAD, and returned the substring between "from" and
"to", but there was no way, if the substring was a 40-hex string, to
tell if we were on a detached HEAD at that commit object, or on a
branch whose name happened to be the 40-hex string.

At this point, we cannot afford to change the format recorded in the
reflog, so introduce a heuristics to see if the 40-hex matches the
object name of the commit we are switching out of.  This will
unfortunately mishandle this case:

	HEX=$(git rev-parse master)
	git checkout -b $HEX master
	git checkout master

where we were indeed on a non-detached $HEX branch (i.e. HEAD was
pointing at refs/heads/$HEX, not storing $HEX), of course, but
otherwise should be fairly reliable.

Signed-off-by: Junio C Hamano <gitster@xxxxxxxxx>
---

 * This is a preparatory step for the beginning of a much larger
   series.  Peff is Cc'ed because one of the most tricky issue
   involves what d46a8301930a (fix parsing of @{-1}@{u} combination,
   2010-01-28) did.

 sha1_name.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/sha1_name.c b/sha1_name.c
index 3820f28..1473bb6 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -862,6 +862,7 @@ static int get_sha1_oneline(const char *prefix, unsigned char *sha1,
 struct grab_nth_branch_switch_cbdata {
 	int remaining;
 	struct strbuf buf;
+	int detached;
 };
 
 static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
@@ -880,9 +881,14 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 	if (!match || !target)
 		return 0;
 	if (--(cb->remaining) == 0) {
+		unsigned char sha1[20];
+
 		len = target - match;
 		strbuf_reset(&cb->buf);
 		strbuf_add(&cb->buf, match, len);
+		cb->detached = (len == 40 &&
+				!get_sha1_hex(match, sha1) &&
+				!hashcmp(osha1, sha1));
 		return 1; /* we are done */
 	}
 	return 0;
@@ -891,8 +897,12 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 /*
  * Parse @{-N} syntax, return the number of characters parsed
  * if successful; otherwise signal an error with negative value.
+ * The string in buf.buf is either a branch name (needs to be
+ * prefixed with "refs/heads/" if the caller wants to make it
+ * a fully spelled refname) or 40-hex object name of the detached
+ * HEAD, and *detached is set to true for the latter.
  */
-static int interpret_nth_prior_checkout(const char *name, struct strbuf *buf)
+static int interpret_nth_prior_checkout(const char *name, struct strbuf *buf, int *detached)
 {
 	long nth;
 	int retval;
@@ -917,6 +927,8 @@ static int interpret_nth_prior_checkout(const char *name, struct strbuf *buf)
 	if (0 < for_each_reflog_ent_reverse("HEAD", grab_nth_branch_switch, &cb)) {
 		strbuf_reset(buf);
 		strbuf_add(buf, cb.buf.buf, cb.buf.len);
+		if (detached)
+			*detached = cb.detached;
 		retval = brace - name + 1;
 	}
 
@@ -992,7 +1004,7 @@ int interpret_branch_name(const char *name, struct strbuf *buf)
 	char *cp;
 	struct branch *upstream;
 	int namelen = strlen(name);
-	int len = interpret_nth_prior_checkout(name, buf);
+	int len = interpret_nth_prior_checkout(name, buf, NULL);
 	int tmp_len;
 
 	if (!len)
-- 
1.8.3-rc1-182-gc61d106

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